The present invention relates to a process for preparing a rubber sheet in which short fiber is oriented in the circumferential direction of tube shaped rubber. Also, the present invention relates to a studless tire superior in performance on snow and ice and a process for preparing the tread of the studless tire.
In recent years, studless tires without spikes have come to be widely used as tires for driving on icy roads. In a studless tire, in order to improve performance on ice, road digging friction and adhesion friction must be increased. Therefore, various studies have been conducted to raise the friction coefficient of tread rubber to icy roads.
Also, a tire which uses as the tread rubber, a rubber in which short fiber (organic short fiber or inorganic short fiber) is compounded, is known to improve the strength, rigidity and abrasion resistance of a tire.
JP-A-2001-315504 suggests a method for preparing a tread by rolling a rubber composition in which short fiber is dispersed with a calender roll and folding the obtained sheet. However, when tread rubber is extrusion molded by a calender roll or extruder, the compounded short fiber is oriented in the extrusion direction, that is along the circumferential direction of the tire tread. As a result, the road scratching effect of short fiber does not function effectively in a large part of the tread rubber which touches the road and so the tire tread of this method is hardly used for a studless tire which requires high digging friction.
Also, there is a method in which short fiber compounded rubber containing short fiber with improved road scratching effect is used in a studless tire. Japanese Patent No. 2637887 suggests using thick short fiber having a diameter of 0.1 to 0.3 mm and a low aspect ratio as the short fiber. When thick short fiber is used, orienting the short fiber when extruding is difficult and so compared with conventional rubber in which short fiber is oriented in the tire circumferential direction, the chances of the ends of the short fiber coming into contact with the road increase, improving the scratching effect to a certain degree. However, the scratching effect is improved only to the degree in which orientation of the short fiber is lost and sufficient performance on ice cannot be obtained.
JP-A-2000-168315 suggests a method for improving the road scratching effect of short fiber and grip performance on ice by orienting short fiber perpendicularly to the tread surface (tread thickness direction). In this method, short fiber is oriented in the pushing direction near the siping by pushing a knife blade into an unvulcanized tread when vulcanizing the tire.
However, in all of the above methods, the orientation degree of short fiber is insufficient or producing a large number of tires is difficult in reality.
In this way, a tire excellent in performance on ice and snow, in which adhesion friction, digging friction and scratching friction on icy and snowy roads are simultaneously improved or obtained in a balanced manner, is currently still not available.